Usually, a polyurethane foam is prepared by mixing and stirring a polyisocyanate and a polyol mixture comprising a polyol, a catalyst, a blowing agent, a foam stabilizer (a surfactant), and other optional auxiliaries, and thereby allowing the components to react.
Shoe soles made of polyurethane foam exhibit excellent properties such as lightness, impact resistance, chemical resistance, abrasion resistance, and bending resistance. With respect to midsoles, in particular, there has recently been a rapid trend toward density reduction for the purposes of reduction in cost and weight. Density reduction in a water-blown polyurethane foam for shoe soles is normally achieved by increasing the relative amount of water in the polyol mixture to increase the generation of CO.sub.2 gas for foaming.
However, increasing the relative amount of water as a blowing agent markedly deteriorates the foam's mechanical properties due to increased urea linkage. Deterioration of split tear strength, a particularly important mechanical property of midsoles, is a problem. Although many attempts have been made to improve the split tear strength through investigation of polyols, prepolymers, or catalysts, no useful means to improve the split tear strength has been found.
On the other hand, an integral skin foam method is used to mold automobile interiors without affecting the foam's mechanical properties. It is characterized in that the skin layer and inner foam are simultaneously molded. This can be achieved by suppressing the expansion of the skin portion which comes into contact with the mold's inner wall during the reaction.
However, because the skin layer is a high-density elastomer, reducing the molded density of the foam as a whole is limited. Also, a chlorofluorocarbon-type blowing agent is required for the integral skin method, which causes various problems in a process where water is used as a blowing agent.
Further, EP-A-0559216 discloses a process for producing a polyurethane integral skin foam by using no halogenated hydrocarbon as a blowing agent.
However, this document does not disclose a method for producing a polyurethane foam having a certain cell diameter distribution.